Every ARM licensee does this though; they license the core designs from ARM and add features (including additional instructions) around it to package into an SOC. It’s just that Apple has the scale to design their own SOCs instead of buying one from Qualcomm or Samsung.
Which most - there is most as in number of cores shipped, and most as in number of organizations who have a license.
The second I have no doubt you are correct - I know of several organizations that have licensed ARM just to ensure they have a long term plan to get more without the CPU going obsolete again (one company has spent billions porting software that was perfectly working on a 16 bit CPUs that went obsolete - there was plenty of CPU for any foreseeable feature, but no ability to get more). These want something standard - they are kind of hoping that they can combine a production run with someone else in 10 years when they need more supply and thus save money on setup fees.
The first is a lot harder. The big players ship a lot of CPUs, and they the volumes to make some customization for their use case worth it. However I don't know how to get real numbers.
